Method and apparatus for determining the titratable acidity of spent sulfuric alkylation acid



March 16, 1965 S. F. METHOD AND APPARATUS FOR Filed NOV. 17, 1961 KAPFF3, DETERMINING THE TITRATABLE ACIDITY 0F SPENT SULFURIC ALKYLATION ACID3 Sheets-Sheet 1 Sump 3g 39.. v V v 3.9 FILTER Wafer J] V 32 27 K L w 4326' /INVNTOR.

.Sixr Frederick Kapff ZZZzW W ATTORNEY I March 16, 1965 KAPFF 3,173,969

S. F. METHOD AND APPARATUS FOR DETERMINING THE TITRATABLEI ACIDITY OFSPENT SULFURIC ALKYLATION ACID Filed Nov. 17, 1961 3 Sheets-Sheet 2 Fig.2

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Sixf Frederick Kapff ATTORNEY United States Patent h/ ETHOD ANDAPPARATUS FOR DETERMINING THE TITRATABLE ACIDITY 0F SPENT SUL- FURICALKYLATION ACE) Sixt Frederick Kapif, Homewoed, Ill, assignor toStandard Oil Company, Chicago, Ill., a corporation of Indiana Filed Nov.17, 1961, Ser. No. 153,159 27 Claims. (Ci. 260-68362) This inventionrelates to determining the concentration of spent sulfuric acid employedin sulfuric acid catalyzed alkylation processes. More particularly, thisinvention relates to a method and means for measuring the titratableacidity of spent acid employed in the sulfuric acid alkylation ofisoparaflin hydrocarbons with olefin hydrocarbons. More specifically,this invention relates to a method and means for measuring theconcentration of such spent sulfuric acid by deter-mining changes in theelectrical conductivity of a progressively diluted spent sulfuric acidsample.

The method of alkyla-ting isoparafiins and olefins in the presence of aconcentrated sulfuric acid catalyst is well known in the petroleumrefining art. Sulfuric acid alkylation processes have been described indetail in a number of publications such as the article Alkylation WhatYou Should Know" published in the Petroleum Refiner, volume 37, No. 9,pages 316-329, September 1958. Typically, in this process, isobutane andbutylenes are alkylated using a concentrated sulfuric acid catalystwhich may range upwards in strength from about 85 weight percent H 80The acid strength tends to decrease during the operation of the processdue to dilution with su-lfonated hydrocarbons, i.e.,catalyst-hydrocarbon complexes and esters, and with water. As the acidis diluted its activity decreases, especially as a result of waterdilution. In thetypical instance, the alkylation reaction is carried:out in a series of mixing zones without the addition of acid to thematerials passed through the zones. For satisfactory operation the acidstrength in the last reactor must be maintained above a certain minimumconcentration. When the acid strength drops below an establishedminimum, the acid is discarded. It is usual to recycle a portion of thespent alkylation acid inasmuch as is possible to reduce the operatingcosts of the process, since the cost of acid used is important. Thus, itis desirable to minimize the amount of spent acid discarded because ofthe drop in acid strength. However, the determination of the acidstrength, i.e., the titratable acidity, is time consuming and expensivewhen carried out in the laboratory, and laboratory methods do notprovide suiiicient rapidity for close operational control of theprocess.

The general object of the present invention is a method and means formeasuring the strength of spent sulfuric acid alkylati-on catalyst whichprovides rapid and accurate information on the spent acid strength andwhich enables the unit operator to reduce acid consumption.

The principle of measuring the strength of various acids by determiningthe electrical conductivity of the acid solution has been recognized forsome time. For example, it fresh concentrated sulfuric acid is dilutedwith water the electrical conductivity increases rapidly between about98 percent and 92 percent and then drops oif before rising again as thedilution progresses. Thus, the first-occurring maximum in the electricalconductivity is at about a 92 percent by weight acid concentration.However, the electrical conductivities of spent sulfuric acid alkylationcatalysts are very similar, regardless of the true titrat-able acidity,and these approximately correspond in electrical conductivity to a freshacid-Water mixture of about 98 percent acid. If a spent acid sample isprogressively 3,173,959 Patented Mar. 16, 1965 diluted with water theconductivity will increase to a maximum peak and then drop off as in thecase of the diluted fresh acid. Now, it has been found that each of thespent acids has a characteristic maximum conductivity which is relatedto the original titratable acidity of the spent acid before waterdilution. Thus, the maximum electrical conductivity can be determinedfor a spent acid sample which has been progressively diluted with waterand titrata-ble acidity of the undiluted sample can be readilyascertained. The method of obtaining the relationship between the acidstrength and the electrical conductivity characteristics comprisesprogressively diluting a plurality of spent sulfuric alkylation acidsamples of known acid-i ties with Water, the total dilution of eachsample being at least great enough to cause a first maximum inelectrical conductivity to occur therein, measuring the first maximumelectrical conductivity occurring in said samples at a knowntemperature, and correlating the original titratable acidity of eachsample with the first indicated maximum conductivity of each sample.

The present invention further provides a method for determining theconcentration of a spent sulfuric acid alkylation catalyst whichcomprises diluting a sample of spent sulfuric alkylation acid with watersufiicient to cause a first maximum in electrical conductivity to occurin the diluted sample, measuring the electrical conductivity of thediluted sample at a known temperature to determine a value indicative ofthe first-occurring maximum in electrical conductivity for the dilutedsample and determining the ti-tratable acidity of the original sample byreference to a previously established correlation between the knowntitratable aoidities of a plurality of spent sulfuric alkylation acidsand their corresponding values indicative of the firstoccurring maximumelectrical conductivities.

The present invention additionally provides apparatus for determiningthe concentration of spent sulfuric acid alkylation catalyst whichcomprises mixing chamber mean-s, means for introducing a spent sulfuricalkylat-ion acid into said chamber means, means for progressivelydiluting an acid sample in said chamber means with wafor sufficient tocause a first maximum to occur in'the electrical conductivity of thediluted sample, temperature control means for maintaining liquid in saidchamber means at a substantially constant temperature, measuring meansfor measuring the electrical conductivity of liquid in said chambermeans, indicator means connected to said measuring means for receiving asignal from said measuring means and for indicating changes in theelectrical conductivity of the liquid in said chamber means, and meansfor correlating the electrical conductivity characteristics of thediluted sample with the titratable acidity of the original sample.

The present invention will be more fully understood by reference to thefollowing description of a preferred embodiment of apparatus accordingto the present invention and to the accompanying drawings wherein:

FIGURE 1 is a schematic representation of a-preferredembodiment ofapparatus for monitoring the strength of spent sulfuric acid alkylationcatalyst;

FIGURE 2 which is a detailed view of a conductivity cell and cell holderemployed in the apparatus of FIG-" URE 1;

FIGURE 3 wherein it is shown a plot of electrical con ductivity versusthe known titratable acidity for a number of sulfuric acid alkylationcatalysts; and

FIGURE 4 which is a plot of the first-occurring conductivity maximumversus the original undiluted acid strength for a number of spentsulfuric acid alkylation catalysts.

Turning now to FIGURE 3, it is seen from plot A that if a sample offresh sulfuric acid of about 98 percent concentration is progressivelydiluted with increasing amounts of water in increments of about 2percent, the electrical conductivity of the acid rises rapidly to a peakat about 92 percent and then decreases, If diluted further, theconductivity will begin to rise again. However, if a sample of spentsulfuric alkylation acid is diluted in a similar manner the electricalconductivity will exhibit a first maximum at a titratable acidity whichis unique for each spent acid sample. In plots B, C and D, theconductivity characteristics are plotted for spent alkylation acidsamples taken from the alkylation of isobutane and butylenes and havingacid strengths of approximately 94 percent, 92 percent and 88 percent,respectively. From these latter plots it is seen that each spent acidsample exhibits a firstoccurring maximum in electrical conductivitywhich may readily be correlated with the initial titratable acidity ofthe sample. Generally speaking the addition of about 8 percent water,based on sample has been found sufficient to achieve the first-occurringconductivity maximum in the acid samples tested. However, greater orlesser degrees of dilution may be employed, dependent upon theparticular samples run and system employed in testing.

Preferably, the above correlation is performed graphically as shown inFIGURE 4, wherein the values indicative of each conductivity maximum areplotted versus the known original titratable acidity for each of anumber of spent alkylation acid samples. From FIGURE 4 it is seen thatonce the correlation has been made between the firstoccurring maximum inelectrical conductivities and the initial titratable acidities for aseries of acid samples, the original titratable acidity for a sample canbe readily determined by reference to the previously establishedcorrelation,

The correlation shown graphically in FIGURE 4 was established for aseries of acid samples from the alklation of isobutane and butylenes.Similar correlations can readily be established for spent alkylationacids from the sulfuric acid alkylation of other isoparaflins, such asisopentane, isohexane, etc., and olefins, such as propylene, amylene,etc.

Apparatus for monitoring the strength of spent sulfuric alkylation acidcan be utilized to aid the plant operator to more closely control acidstrength. The following description refers to a preferred automatic acidmonitor. As shown in FIGURE 1, spent sulfuric alkylation acid is passedby way of line 11 through valve 12 to the inlet of acid pump 13. At apreselected pressure this pump discharges at a substantially constantflow rate, which typically is about 60 ccs. per minute. The discharge ispassed by way of line 14 through a check valve 16 to the inlet of amixing chamber 17 which is positioned in a constant temperature bath 18employed to remove the heat of mixing and to maintain the diluted sampleat substantially constant temperature, i.e. about 100 Water is passed byway of lines 21 and 23 to the inlet of a dilution pump 24 which is avariable discharge pump, which typically has a discharge rate varyingbetween about 0 and 16 ccs. per minute at about 30 p.s.i. The dischargefrom pump 24 is passed by way of line 26 through a check valve 27 and byway of line 28 to the inlet of the mixing chamber 17 wherein the waterintimately contacts and dilutes the incoming acid.

The continuously flowing acid sample is progressively diluted in themixing chamber 17 by the water admitted thereinto and the mixture passesfrom the chamber 17 by way of line 29 to the inlet end of an electricalconductivity cell 31, also positioned in the constant temperature bath.The conductivity cell is of a type commercially available and comprisestwo spaced-apart electrodes 71 and 72, as shown in FIGURE 2. Theconductivity cell is positioned within the cell holder 73 as shown. Theliquid from line 29 enters the cell inlet 74 and passes through theconductivity cell to the cell outlet 75 whilie an electrical current ispassed between the two electrodes. The conductivity cell is positionedwithin the cell holder by means of the insulating inserts 76 and 77,which, typically, are made of tetrafluoroethylene (Teflon) resin. Afluid tight seal is accomplished at the outlet end of the cell by meansof 0 rings 79 and 80, also made of Teflon. The inlet chamber 82 in thecell holder is provided as an additional protection against bubblesentering the conductivity cell and openings 83 are provided in theinsert 76 to bleed 01f any trapped gases and to permit them to bypassthe measuring system. Further, a means of pressure control is connectedto the cell outlet by line 4,5 to maintain a constant pressure in thesystem and to prevent gas bubbles from forming in the acid-watermixture, since bubbles result in erratic readings in the conductivitycell. The pressure controller 40 employs a vertical inlet extendingupwardly into the controller chamber and a weighted mass is fitted overthe discharge end of the inlet tube to maintain the system pressure atabout 30 p.s.i. This pressure has been found suflicient to prevent theformation of gas bubbles.

As mentioned above, the rate of water flow to the mixing chamber iscontrolled by pump 24. For this purpose a pneumatic controller 32 isemployed, and, upon a signal on a timer (not shown), solenoid valve 33opens which bleeds the air chamber 34 down to about 3 p.s.i., as limitedby pressure relief valve 36. The valve 33 next connects the chamber 34to a source of pressured air, the pressure of which is regulated bymeans of pressure regulator 37 (about 15 p.s.i.) and air then flows intothe chamber 34 at a rate which is determined by the setting of the valve38 which acts as a variable orifice. The air flows into the air chamberfor a predetermined time after which the solenoid valve 33 again opensand the cycle is repeated. As the pressure in the chamber 34 builds up,the pneumatic pressure is transmitted by way of line 39 to the pneumaticcontroller 32 and acts to open the discharge of the pump 24 toprogressively increase the discharge rate from the dilution pump. Thetiming is such that the maximum water flow rate from the pump 24 isabout 16 ccs. per minute and the total cycle time is about 20 minutes.

The circular chart recorder 41 is electrically connected by lead means42 to the conductivity cell 31 to receive an electrical signal from theconductivity cell and to continuously provide a graphic record of thechanges in the electrical conductivity of the sample-water mixtures asthe sample is diluted with greater amounts of water. If de sired, thechart may be printed to read directly the initial titratable acidity ofthe acid sample.

Water is supplied by way of lines 21 and 43 to a heat exchanger coil 44in the constant temperature bath to remove heat from the bath andprovide a means of achieving substantially constant temperature controlas described above. The amount of water flowing through the coil 44 maybe regulated by the valve 46 in the outlet 47 connected to the coil.Additionally, water is supplied by way of line 48, with the flow ratebeing controlled by valve 49, to the pressure control valve 40 toprovide suflicient flow through the pressure control device for optimumoperation of the apparatus. Further, flushing water may be supplied asdesired by way of line 51 to the inlet of the acid pump 13 so that thesystem may be flushed out with water to remove acid trapped therein.

The above-described apparatus may be constructed of any suitablecorrosion-resistant material such as Teflon or other resins and alloyssuch as Hastelloy B, Hastelloy D or Carpenter 20.

The foregoing description of the preferred embodiment of the presentinvention is given for illustrative purposes only. From the foregoingdescription various modifica tions in the construction and operation ofapparatus for practicing the present invention will become apparent tothe skilled artisan, and, as such, these fall within the spirit andscope of the present invention. For example, the water may be suppliedto the mixing chamber through a series of timer-actuated solenoid valvesarranged in parallel and operated by a timer program to be successivelyopened and change the water addition in approxiaivaeeb mately 2 percentincrements of 0 to about percent water. Likewise, the above-describedpumping system may be employed using a variable speed dilution pump :toachieve theincremental dilution of the sample. Similarly, variousbatch-type systems can be devised to accomplish the results obtainablewith the above-described apparatus. 7

What I claim is:

1. The method of obtaining a relationship between the titratable acidityof spent sulfuric acid alkylation catalyst and the electricalconductivity thereof which method comprises progressively diluting aplurality of spent sulfuric alkylation acid samples of known aciditieswith water, the total dilution of each sample being at least greatenough to cause a first maximum in electrical conductivity to occurtherein, measuring the first maximum electrical conductivity occurringin said samples at a known temperature, whereby said first maximumelectrical conductivity provides a measure of the original titratableacidity of each sample.

2 The method of claim 1 wherein said dilution is varied from about 0weight percent to at least about 8 weight percent, based on sample.

3. The method of claim 1 wherein said acid sample is taken from aprocess for the alkylation of isobutane and butylenes.

4. The method of determining the titratable acidity of a spent sulfuricacid alkylation catalyst which comprises diluting a sample of spentsulfuric alkylation acid with Water sufiicient to cause a first maximumin electrical conductivity to occur in the diluted sample, measuring theelectrical conductivity of the diluted sample at a known temperature todetermine a value indicative of the first-occurring maximum inelectrical conductivity for the diluted sample whereby said valueprovides a measure of the titratable acidity of said spent sulfuric acidalkylation cataylst.

5. The method of claim 4 wherein said spent acid sample is taken from aprocess for the alkylation of isobutane and butylenes.

6. The method of claim 4 wherein said spent acid sample is continuouslypassed through a mixing chamber at a substantially constant flow rate,said water is passed through said mixing chamber at a flow rate whichincreases substantially continuously from about 0 percent to at leastabout 8 percent of said sample flow rate to intimately contact saidsample in said chamber, and wherein said conductivity measurement ismade substantially continuously.

7. The method of regulating the acid strength in the sulfuric acidalkylation of isoparaffins with olefins which method comprisesvvdthdrawing a sample of spent sulfuric acid from the alkylationoperation, progressively dilut ing the withdrawn sample with watersufiicient to cause a first maximum in electrical conductivity to occurin the diluted sample, measuring the electrical conductivity of thediluted sample at a known temperature to determine the first-occurringmaximum in electrical conductivity, whereby said first-occurring maximumprovides a measure of the original titratable acidity of said spentsulfuric acid, and employing said original tn'tatable acidity to controlthe discard of spent acid from said alkylation process.

8. The method of claim 7 wherein said dilution progresses from about 0percent to at least about 8 percent, based on sample.

9. The method of claim 7 wherein said process is the alkylation ofisobutane with butylenes.

10. The method of claim 8 wherein said dilution is continuous andwherein said conductivity measurement is made continuously during saiddilution.

11. Apparatus for determining the concentration of spent sulfuric acidallcylation acid which apparatus comprises a mixing chamber, means forintroducing spent sulfuric alkylation acid into said chamber, means forprogressively diluting an acid sample in said chamber with water,temperature control means for maintaining liquid in said chamber at asubstantially constant temperature, measuring means for measuring theelectrical conductivity of liquid in said chamber, indicator meansconnected to said measuring means for receiving a signal from saidmeasuring means and for indicating changes in the electricalconductivity of the liquid in said chamber.

12. The apparatus of claim 11 wherein said dilution means supplies waterin an amount increasing from about 0 percent to at least about 8percent, based on sample, to said chamber.

13. The apparatus of claim 11 wherein said sample introduction means isa constant flow rate pump and said dilution means is a variabledischarge pump having a flow rate variable between about 0 and at leastabout 8 percent of the flow rate of said constant flow rate pump.

14. The apparatus of claim 11 wherein pressure control means areprovided to maintain a preselected pressure in said mixing chamber.

15. Alkylation acid monitor apparatus which comprises a mixing chamberprovided with inlet means and outlet means, an acid pump connected tosaid inlet means for passing an acid sample through said chamber at asubstantially constant rate of flow, a dilution pump having a variabledischarge rate connected to said inlet means for progressively dilutingliquid in said chamber, means for varying the discharge rate of saiddilution pump from 0 to a least about 8 percent of the flow rate of saidacid pump, an electrical conductivity cell connected to said outletmeans to receive liquid from said chamber, a recorder electricallyconnected to said cell to provide a graphic record of the electricalconductivity characteristics of liquid passed through said cell, aconstant temperature bath for maintaining the liquid in said chamber andin said cell at a substantially constant temperature and means forcorrelating the electrical conductivity characteristics of the dilutedsample with the titratable acidity of the original sample.

16. The apparatus of claim 15 wherein there is provided a pneumaticvalve on said dilution pump to control the discharge rate therefrom andmeans for varying the pneumatic pressure supplied to said valve tocontrol said discharge rate according to a preselected timed schedule.

17. The apparatus of claim 15 wherein there is provided pressure controlmeans connected to said chamber to maintain the pressure therein at apreselected level.

18. Apparatus which comprises a mixing chamber provided with inlet meansand outlet means, a constant flow rate sample pump connected to saidinlet means, a variable discharge rate dilution pump connected to saidinlet means, means for varying the discharge rate of said dilution pumpfrom 0 to a least about 8 percent of the flow rate of said sample pumpaccording to a preselected timed schedule, an electrical conductivitycell provided with an inlet and an outlet and connected to said chamberoutlet means to receive liquid from said chamber, a pressure controllerconnected to said cell outlet for maintaining the pressure therein at apreselected level, a constant temperature bath for maintaining liquid insaid chamber and in said cell at a substantially constant temperature,and a recorder electrically connected to said cell to provide a graphicrecord of the electrical conductivity characteristics of liquid passedthrough said cell.

19. A method for determining the titratable acidity of spent sulfuricacid alkylation catalyst which comprises progressively diluting saidspent alkylation catalyst with water, and determining the electricalconductivity of said catalyst during said diluting, whereby the changeof said electrical conductivity during said diluting provides a measureof the titratable acidity of said spent sulfuric acid alkylationcatalyst.

20. The method of claim 19 wherein said spent sulfuric acid alkylationcatalyst contains more than about percent by Weight of sulfuric acid.

21. The method of claim 19 wherein the pressure of the catalyst ismaintained sufiiciently high while determining the electricalconductivity to prevent bubble formation in said catalyst.

22. The method of claim 19 wherein the amount of water used for saiddiluting is within the range of -20 ,percent by weight based on saidspent sulfuric acid alkylation catalyst.

23. The method of determining the titratable acidity of spent sulfuricacid alkylation catalyst which comprises progressively diluting a sampleof said alkylation catalyst with Water suflicient to cause a firstmaximum in electrical .conductivity to occur in the diluted sample, andmeasuring the electrical conductivity of the sample during saiddilutisoparaflins with olefins, which method comprises withdrawing asample of said catalyst, progressively diluting said sample with watersufficient to cause a first-occurring maximum electrical conductivity ofsaid sample, measuring the electrical conductivity of said sample duringsaid diluting to determine said first-occurring maximum electricalconductivity, and controlling the fresh sulfuric acid addition rate tosaid process in response to said determined first-occurring maximumelectrical conductivity to maintain said first-occurring maximumelectrical conductivity at a value corresponding to the desiredpredetermined titratable acidity of said catalyst.

26. The method of claim wherein the diluting-water concentration in saidsample is varied within the range of 0-2O percent, based on said sample.

27. The method of claim 25 wherein the measurement of electricalconductivity is carried out at a substantial- 1y constant temperature.

References Cited in the file of this patent UNITED STATES PATENTS2,592,063 Persyn Apr. 8, 1952 2,765,218 Arnir Oct. 2, 1956 2,977,199Quittner Mar. 28, 1961

1. THE METHOD OF OBTAINING A RELATIONSHIP BETWEEN THE TITRATABLE ACIDITYOF SPENT SULFURIC ACID ALKYLATION CATALYST AND THE ELECTRICALCONDUCTIVITY THEREOF WHICH METHOD COMPRISES PROGRESSIVELY DILUTING APLURALITY OF SPENT SULFURIC ALKYLATION ACID SAMPLES OF KNOWN ACIDITIESWITH WATER, THE TOTAL DILUTION OF EACH SAMPLE BEING AT LEAST GREATENOUGH TO CAUSE A FIRST MAXIMUM IN ELECTRICAL CONDUCTIVITY TO OCCURTHEREIN, MEASURING THE FIRST MAXIMUM ELECTRICAL CONDUCTIVITY OCCURRINGIN SAID SAMPLES AT A KNOWN TEMPERATURE, WHEREBY SAID FIRST MAXIMUMELECTRICAL CONDUCTIVITY PROVIDES A MEASURE OF THE ORIGINAL TITRATABLEACIDITY OF EACH SAMPLE.